This invention relates generally to methods and apparatus for utilizing or recovering the heat content in waste water and, more particularly, to a method and apparatus for utilizing or recovering by means of a heat pump, the heat content in waste water which has passed through a biological purification plant in which the last step comprises sedimentation of the waste water in a settlement tank. According to the invention, the evaporator section of the heat pump is heated by means of the purified waste water subsequent to sedimentation.
It is well known that in order to operate a heat pump in an economic manner, it is necessary to operate the heat pump at substantially its full capacity. This condition requires that the capacity of the heat pump, when utilized in conjunction with a heat source having a varying output, not substantially exceed the minimum output of the heat source. In a biological waste water purification plant, the rate of flow of the waste water normally follows a curve which as a function of time has the approximate shape of a sine curve, the minimum flow rate being about 1/4 of the maximum flow rate. Thus, when the capacity of the heat pump corresponds to the minimum flow rate of the waste water through the purification plant, only about 1/2 of the average heat from the heat source can be utilized.
Although it is possible to store the waste water in a reservoir in order to reduce the daily flow variations, such storage is practically precluded both for economic reasons and due to space requirements.